Problem: Stephanie is 2 times as old as Luis. Six years ago, Stephanie was 5 times as old as Luis. How old is Luis now?
Explanation: We can use the given information to write down two equations that describe the ages of Stephanie and Luis. Let Stephanie's current age be $s$ and Luis's current age be $l$ The information in the first sentence can be expressed in the following equation: $s = 2l$ Six years ago, Stephanie was $s - 6$ years old, and Luis was $l - 6$ years old. The information in the second sentence can be expressed in the following equation: $s - 6 = 5(l - 6)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $l$ , it might be easiest to use our first equation for $s$ and substitute it into our second equation. Our first equation is: $s = 2l$ . Substituting this into our second equation, we get: $2l$ $-$ $6 = 5(l - 6)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $2 l - 6 = 5 l - 30$ Solving for $l$ , we get: $3 l = 24.$ $l = 8$.